Crystal structure solution and prediction via global and local optimization

Simulations machinery broadly available today allows direct-space solutions of crystal structures based on high quality powder diffraction data for sy stems with up to some 15-20 degrees of freedom. Analogous machinery for pre dicting new structures is rarely unsuccessful, but has had limited systemat ic application. The prime challenges for structure solution are for degrees of freedom beyond some 15-20, and for powder diffraction data which is of insufficient quality to allow an unambiguous indexing. In structure predict ion, the corresponding issues are (i) enumeration strategies, (ii) collated access to the results of prior predictions, (iii) relating results in some fashion to likelihood of occurrence, encompassing finite temperature issue s, and (iv) rational approaches to the synthesis of designed structures. Re cent progress in the field has been steady and, while solutions to these ke y problems are not immediately obvious, the pace of progress over the past 5 years suggests major headway is imminent in a number of these areas. (C) 2000 Elsevier Science Ltd. All rights reserved.